Power is a word we use all the time, but do you really know what it means? We'll look at how electrical power relates to voltage, current and resistance and how knowing that relationship can help you in everyday life.
Power is a measure of how many joules are used/unit of time
I recently moved into my new house, and the first thing I planned to do was to turn one of the rooms into a home theater. I'd been dreaming of this for a long time and was excited to get started. I was talking to a friend of mine who happened to be an electrician, and he warned me that I needed to be careful when choosing the equipment so that I didn't overload the circuit breakers. Heeding his warning, I went home and checked the rating of the circuit breaker for my home theater room. It said 15 amps.
Okay, I thought; now I know that together the equipment can draw no more than 15 amps of current. I got out the home theater catalog and started looking at TVs and surround sound systems. I immediately saw a problem. All of the equipment was rated in watts of power and not amps of current! I knew that power and current were related in some way, but I wasn't sure how.
Power And Watts
In search of answers, I dug out my old physics textbook and started reading. It turns out that power is a measure of how much energy is used over a period of time. There are many different units that can be used to express power, such as horsepower for cars. However, electrical power is almost universally given in watts. One watt is equal to one joule per second. This made sense when I considered that power is a measure of how much energy, measured in joules, is used per unit of time, like a second. I guess it's just easier to say watts instead of joules per second.
Power, Current And Voltage
Formula for finding the power using voltage
All this new information was great, but I still wasn't sure how to solve my problem, so I kept reading. The book went on to say that electrical power used by a device can be calculated by multiplying the current by the difference between the voltage going into and out of the device. Now I knew I was getting somewhere.
The home theater equipment was rated in watts of power and the breaker in amps of current, so all I was missing was the voltage. I knew that for devices plugged into an electrical outlet, the voltage difference through the device was equal to the voltage difference between the two slots of the outlet. Using my electrical meter, I carefully probed the electrical outlet and measured 120 volts.
With all the pieces of the puzzle, I multiplied 120 volts by the maximum current of the circuit breaker, 15 amps, and came up with 1800 watts of power. This meant that I had to select a TV and surround sound system that together used no more than 1800 watts of power.
Power, Current And Resistance
Later that day, my electrician friend came over to see how I was coming along with picking out equipment. Excitedly, I showed him the gigantic 80-inch flat screen TV and mega-bass surround sound system I had selected from the catalog. I assured him that I had done my research and figured out that this equipment was just under the 1800-watt limit that I had calculated. 'Not so fast,' he said; 'You're forgetting something! You're not accounting for the power lost in the house wiring before it even gets to your equipment.' He was right. I hadn't even thought of this.
Due to the resistance of the wires running from the circuit breaker to the home theater room, some of the electrical energy would be converted to heat, reducing the amount of power available at the outlet. I needed to figure out how much power would be lost, but there was a problem. I couldn't measure the voltage difference from one end of the wire to the other unless there were 15 amps of current flowing through it. Unfortunately, I didn't have any way to do that. Desperate for answers, I went back to my physics book.
Formula for determining power using resistance
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